(1) Field of the Invention
The invention relates to the fabrication of integrated circuit devices, and more particularly, to a method of optimizing pre-layer film thickness by using Chemical Mechanical Polishing (CMP) end-point detection signals.
(2) Description of the Prior Art
The continued emphasis on semiconductor device miniaturization, leading to the technological evolution of Large Scale Integration (LSI), Very Large Scale Integration (VLSI) and Ultra Large Scale Integration (ULSI), has over the years resulted in inter-linear device distances that have become increasingly short. One of the predominant technologies that is used for the creation of semiconductor devices is the technology of photolithography which uses, with and in support of other developments in the semiconductor industry, ever shallower focal depths to create images in target surfaces. As a result of this ever shallower image depth, the target surfaces must be created with increased flatness. If a large step (an abrupt change in the planar geometry of the surface) is present in the semiconductor surface, step coverage is negatively affected. This results in a gradation of the surface that is too severe and causes poor deposition of for instance an overlying layer of metal, making it difficult to achieve a reliable semiconductor device. Increased semiconductor device density is frequently implemented using multi-layered configurations, which further leads to demands of increased planarity of the surface over which additional semiconductor device features are created. One of the main techniques that have been used in the semiconductor industry to achieve optimum surface planarity is the method of Chemical Mechanical Polishing (CMP). Chemical Mechanical Polishing is a method of polishing materials, such as semiconductor substrates, to a high degree of planarity and uniformity. The process is used to planarize semiconductor slices prior to the fabrication of semiconductor circuitry thereon, and is also used to remove high elevation features created during the fabrication of the microelectronic circuitry on the substrate.
A CMP apparatus comprises a rotating polishing platen on which a polishing pad is mounted. A second rotating part, typically referred to as the wafer carrier part, holds a wafer. The wafer is frequently attached to the wafer carrier part by means of a clamp ring, the surface of the thus clamped wafer is the surface that is to be polished. The surface of the rotating wafer is brought in intimate contact with the rotating polishing pad while a slurry is distributed over the surface that is being polished. The chemical slurry, which may also include abrasive materials therein, is maintained on the polishing pad to modify the polishing characteristics of the polishing pad in order to enhance the polishing of the substrate.
The use of chemical mechanical polishing to planarize semiconductor substrates has not met with universal acceptance, particularly where the process is used to remove high elevation features created during the fabrication of microelectronic circuitry on the surface of the substrate. One primary problem which has limited the use of chemical mechanical polishing in the semiconductor industry is the limited ability to predict, much less control, the rate and uniformity at which the process will remove material from the substrate surface. As a result, CMP is a labor-intensive process because the thickness and uniformity of the substrate must be constantly monitored to prevent over-polishing or inconsistent polishing of the substrate surface.
One factor, which contributes to the unpredictability and non-uniformity of the polishing rate of the CMP process, is the non-homogeneous replenishment of slurry on the surface of the substrate and the polishing pad. The slurry is primarily used to enhance the rate at which selected materials are removed from the substrate surface. As a fixed volume of slurry in contact with the substrate reacts with the selected materials on the surface of the substrate, this fixed volume of slurry becomes less reactive and the polishing enhancing characteristics of that fixed volume of slurry is significantly reduced. One approach to overcoming this problem is to continuously provide fresh slurry onto the polishing pad.
This approach presents at least two problems. Because of the physical configuration of the polishing apparatus, introducing fresh slurry into the area of contact between the substrate and the polishing pad is difficult. Providing a fresh supply of slurry to all positions of the substrate is even more difficult. As a result, the uniformity and the overall rate of polishing are significantly affected as the slurry reacts with the substrate.
FIG. 1 shows a Prior Art CMP apparatus. A polishing pad 12 is affixed to a circular polishing table 16 which rotates in a direction indicated by arrow 20 at a rate in the order of 1 to 200 RPM. A wafer carrier 14 is used to hold wafer 10 face down against the polishing pad 12. The wafer 10 is held in place by applying a vacuum to the backside of the wafer (not shown). The wafer carrier 14 also rotates as indicated by arrow 19, usually in the same direction as the polishing table 16, at a rate on the order of 1 to 200 RPM. Due to the rotation of the polishing table 16, the wafer traverses a circular polishing path over the polishing pad 12. A force 18 is also applied in the downward vertical direction against wafer 10 and presses the wafer 10 against the polishing pad 12 as it is being polished. The force 18 is typically in the order of 0 to 15 pounds per square inch and is applied by means of a shaft 16 that is attached to the back of wafer carrier 14. Slurry 13 is provided to the top of the polishing pad 12 to further enhance the polishing action of polishing pad 12.
The Prior Art method that has been highlighted in FIG. 1 encounters a number of problems and concerns. For instance, abrasive polishing particles are lodged in and held by the polishing pad. By using the polishing pad, the fibers of the polishing pad deteriorate, causing the abrasive particle retention within the polishing pad to diminish, reducing the polishing characteristics of the polishing pad. Also, due to the pressure that is applied to the wafer, the contact between the polishing pad and the wafer is intense and does not allow for an even distribution of the polishing slurry across the surface that is being polished. Also, the abrasive particles that essentially affect the polishing action are, during the polishing process, reduced in size, further affecting the polishing characteristics of the process.
FIG. 2a and 2b give and example of the application of the process of CMP in the creation of a semiconductor device. The example that is shown in FIGS. 2a and 2b is characterized by its simplicity and can therefore readily be extended to more complex CMP applications. For all of these applications, the operation is essentially as the example that has been shown in FIGS. 2a and 2b. Over a semiconductor surface 10, for instance the surface of a silicon substrate, a layer 22 of for instance silicon nitride is deposited, patterned and etch, creating openings or trenches through the layer 22 and into the underlying semiconductor surface. These trenches have been filled with an overlying layer 24, for instance comprising silicon oxide. It is the objective to fill the trenches that have been created in layer 22 and the underlying semiconductor surface with a material. To achieve this objective, it is clear from FIG. 2a that the layer 24 must be removed from above the plane that contains the surface of layer 22. CMP is therefore applied to the surface of layer 24, removing this layer starting from the surface of the layer 24 and proceeding until the surface of layer 22 is reached. The results of the CMP process is shown in FIG. 2b, indicating that the objective of filling the trenches that have been created through layer 22 and into the surface 10 has been achieved. Numerous variations of the simplified example, shown in FIGS. 2a and 2b, of a processing environment where CMP can be applied need not be further detailed at this time, since these variations may change in particulars but not in the essential process as shown in FIGS. 2a and 2b. 
Where FIGS. 1, 2a and 2b have shown, in principle and in simplified form, the methods that are used to affect surface polishing, an equally important aspect of the CMP process is to observe the status and results that are obtained during and after the process of CMP. The invention provides a method that addresses semiconductor surface polishing during and at the end of the process of CMP.
U.S. Pat. No. 6,074,517 (Taravade) shows a method and apparatus for detecting CMP end-points by an infrared signal.
U.S. Pat. No. 6,020,264 (Lustig et al.) show a CMP thickness determining method.
U.S. Pat. No. 5,985,679 (Beman) shows a CMP end-point detection system.
U.S. Pat. No. 5,868,896 (Robinson et al.) show a CMP Method and end-point system.
It is an objective of the invention to determine a xe2x80x9cbest-can-doxe2x80x9d end-point signal curve during the polishing of a film of a particular thickness, whereby the end-point signal curve is characteristic of the thickness of the film that is being polished.
It is another objective of the invention to make use of monitoring the current that is used to drive a rotating part that is used to polish a semiconductor surface and to therefrom derive a current vs. time curve that is best suited for determining that the CMP end-point has been reached for a layer of a particular thickness.
A new method is provided for determining the optimum film thickness of a film that is to be deposited over a semiconductor surface. The invention observes the electrical current and the therefrom resulting torque that is supplied to a rotating part of a polishing apparatus, from this the CMP end-point can be determined for a reference film that has been deposited. This technique is known as the xe2x80x9cCMP end-point detectionxe2x80x9d technique. The invention addresses observing CMP end-point curves for films of various thicknesses and compares these CMP end-point curves of one film thickness with each other and calculates a deviation for multiple layers (deposited on different wafers) of that film thickness. The process is repeated for different film thicknesses. The film thickness that has a deviation of the CMP end-point curve that closest resembles an optimum deviation is the film thickness that is selected as having the optimum thickness for the deposition of that film.